In narrowband radio channels transmitted by portable radio units, varying multiple reception paths cause the signal strength at the receiver to vary. The variation depends on the environment and the motion of the transmitter and receiver relative to features of the environment. Often, the received signal strength of a faded signal can be characterized by a Rayleigh probability density function. The time duration and frequency of occurrence of the nulls depend on carrier frequency and the speed of the portable unit.
A fade may eliminate a large portion of the energy in the signal at the receiver for some time duration. In a digital system, this can lead to burst errors in which the bit error rate becomes very high over a period of time, even though the average signal strength is sufficient to provide a low bit error rate if no fading has occurred.
If two receivers are receiving a signal transmitted by the same portable transmitter, the fading of the envelope of the signal at the two receivers will be uncorrelated if the receivers are separated by more than a few wavelengths. It is less likely that two receivers will experience a fade at the same time than it is that a single receiver will experience a fade. A technique which takes advantage of this fact is spatial diversity where the signals from two or more receivers, with antennas spatially separated, are combined to reduce the effects of fading. More than two receivers, or branches, may be used. As the number of branches increases, the effects of fading are further reduced.
There are a number of techniques which have been used to combine signals from each of the branches. One of the simplest techniques is known as selection diversity. In this technique, a central processor scans all of the branches for the receiver with the highest signal strength. At any moment in time, the central processor takes its input from the branch with the highest signal strength. Further gains can be achieved by coherently combining the signals from each branch, and adjusting the gain from each branch "on-the-fly" to achieve the maximum signal-to-noise ratio out of the combiner. This is known as maximal ratio combining.
In a conventional system in which multiple binary FSK signals are transmitted simultaneously on multiple subchannels, one way of receiving and performing maximal ratio combining has been to utilize a plurality of narrowband receivers tuned to each of the subchannels and fed from each of the branches. Signals from the plurality of receivers have then been fed to a conventional maximal ratio combiner for producing a combined signal for each subchannel. Unfortunately, the complexity and cost of the conventional system increases in proportion to the number of subchannels, making the conventional system undesirable for more than a limited number of subchannels.
Thus, what is needed is an economical method and apparatus for receiving and maximal ratio combining a plurality of radio signals received from a plurality of branches, wherein each of the radio signals comprises a moderate to large number of subchannels.